Why do we hear humming when we are in a quiet place?

After visiting Death Valley (where it is extremely quiet) and after trying earplugs... I have noticed that there is a humming sound in my head when outside noise is blocked.

So this got me thinking about helicopters and how they use a cancelling soundwave to block out the noise generated by the chopper.

if you take 2 opposite sound waves and you combine them together... giving you destructive interference... then you come out with no noise (in theory...right?)

so, what I was thinking... was that because I live in the city...and there is always a noise murmor in the city or busy metropolitan areas... our brain has created an opposite wave to the murmor which is meant to cancel the noise out.

so, when we are in a quiet area... there is no wave to cancel out what our brain is generating (there is no city noise to destructively interfere with what our brain generated)... so therefore we can hear the wave that our brain generated.

this is just a HS physics student's crazy theory... any input is very very welcome!

It's just your auditory system attempting to pick up a sound. The auditory system is actively trying to detect noises, and the neurons randomly fire every now and then.

The same thing happens to your visual system when you sit in a pitch-black room -- your visual system is actively searching for stimuli, and essentially begins making things up.

Your city-noise theory does not hold water -- people who grow up in rural areas also experience this phenomenon. Besides, the active noise-reduction systems you are talking about are tightly closed-loop: they sample the input and very quickly produce an output to null it. The nulling output has to *exactly* match the input to be cancelled for the system to work. You couldn't just generate a "generic nulling wave," record it on tape, and and play it again and again; it would not match, and would generally make more noise than it cancelled.